Not Applicable.
The invention relates to densified carbon and graphite composites, and their method of manufacture. In particular, the invention relates to a method of manufacturing carbon and graphite composites using a gelcasting process.
Products constructed from carbon and/or graphite composites possess high thermal conductivity, significant heat capacity and excellent friction and wear properties. Because of these characteristics, such composites are often used in speciality applications ranging from the heat shields on the leading edges of the Shuttle Orbiter to exit cones for rocket engines. More commonly, however, these composites are used as frictional materials in the braking systems for military and large commercial aircraft where their unique characteristics provide significantly improved braking performance.
The manufacture of carbon and graphite composites is a lengthy process, and generally involves several cycles of densification and carbonization under substantially high pressure levels. The typical process begins by preparing a carbon preform using a hand lay-up of woven carbon fiber fabric or by hot pressing a mixture of chopped carbon fibers and resin. The preform is then densified by repeated liquid impregnation with pitch or resin, or by carbon vapor infiltration. Following densification, the preform is carbonized or graphitized by heating the preform to temperatures in excess of 600xc2x0 C. and as high as 3000xc2x0 C., as described by K. J. Huttinger, xe2x80x9cTheoretical and Practical Aspects of Liquid-Phase Pyrolysis as a Basis of the Carbon Matrix of CFRC,xe2x80x9d in Carbon Fibers, Filaments and Composites, 301, 301-326 (Figueiredo ed., Kluwer Academic Publishers, Boston 1990), and Brian Rand, xe2x80x9cMatrix Precursors for Carbon-Carbon Composites,xe2x80x9d in Essentials of Carbon-Carbon Composites, supra at 67-102. This densification/carbonization process is repeated until the desired density is achieved (normally 1.8 g/cc). Typically, the complete process requires up to five densification/carbonization cycles over a 6 to 9 month period. As such, products containing carbon or graphite composites tend to carry an extremely high cost.
Other methods of producing carbon or graphite composites have recently been developed so as to avoid the high costs and time expense associated with their manufacture. For example, U.S. Pat. No. 5,556,704, Prevorsek, et al., discloses a method for manufacturing carbon-carbon composites by hot pressing a mixture of carbon fiber and carbon precursor material. Essential to this process is the application of a uniaxial compressive force and a lateral restraining force to the mixture during the heating process. The compressive force is generated by a conventional press apparatus, while the lateral restraining force is generated by a hydraulic piston arrangement. Ultimately, the application of these pressure forces, along with other densification/carbonization processes, results in a high density plate or rod structure.
The recently developed methods, however, are limited in that they require the use of a hot press and thus only allow the production of objects maintaining certain shapes suitable for extrusion or hot pressing, i.e., rods or plates. The manufacturer is therefore required to machine the resulting billet to achieve any product having a complex shape or structural feature, such as a threaded part or a turbine rotor. The additional machining, in turn, consumes more time and increases the final cost of the product.
The high cost of carbon and graphite composites has so for restricted their use to aircraft brakes and other relatively cost insensitive and/or performance driven applications. The benefits of these composites, however, may be readily transferred to the commercial sector if the cost of their manufacture was substantially reduced. For example, commercial applications may include clutch and braking systems for heavy trucks, or railroad locomotives and rail cars. The military may also find numerous applications in brakes and clutches on its fighting vehicles, such as tanks, armored cars, and self propelled artillery.
Affordable graphite or carbon objects having highly complex shapes are also desirable. These shapes may include the intricate designs of a turbine blade or a product having a threaded part. Current methods of manufacture, however, are unable to readily produce such objects. An ideal solution to this problem would include a new method which allows the casting of carbon and graphite composites in a mold such as that used in gelcasting technology.
Gelcasting is a traditional process for producing ceramic components having complex or intricate designs. Specifically, gelcasting is a method of molding ceramic powders into wet xe2x80x9cgreenxe2x80x9d products wherein a monomer solution serves as a binder vehicle, and its controlled polymerization in solution is used as a setting mechanism. The resulting green product is of exceptionally high strength and is typically dried to remove water. After drying, the product is normally heated further to burn off the polymer and is sometimes subsequently fired to sinter the product to a higher density.
Gelcasting methods are well known in the art and are described in Janney, U.S. Pat. No. 4,894,194, Janney et al, U.S. Pat. No. 5,028,362, and Janney et al., U.S. Pat. No. 5,145,908; A. C. Young et al. xe2x80x9cGelcasting of Alumina,xe2x80x9d J. Am. Ceram. Soc., 74 [3] 612-18 (1991); (describing the gelcasting of ceramics such as alumina) Mark A. Janney et al. xe2x80x9cGelcast Tooling: Net Shape Casting and Green Machining,xe2x80x9d in Materials and Manufacturing Processes, (1997) (describing the use of a water-based gelcasting system to form parts using H13 tool steel powder); S. D. Nunn et al., xe2x80x9cGelcasting of Silicon Compositions for SRBSN,xe2x80x9d Ceram. trans., 62, 255-62 (1996) (describing the use of an alcohol-based gelcasting system and a water-based gelcasting system to form parts using silicon powder); M. A. Janney, xe2x80x9cGelcasting Superalloy Powders,xe2x80x9d in P/M in Aerospace, Defense and Demanding Applications, 1995 (Metals Powder Industries Federation, Princeton, N.J., 1995) (describing the use of a water-based gelcasting system to form parts), which are all incorporated herein by reference.
The typical gelcasting process involves the formation of a slurry mixture including ceramic powder, a dispersant for the ceramic powder, and a monomer solution. The monomer solution includes one or more monomers, a free-radical initiator, and an aqueous solvent. Upon its combination, the slurry mixture is transferred to a mold where it is heat-treated at a temperature and for period of time sufficient to polymerize the monomer(s) and form a firm polymer-water gel matrix. The resulting green product is then heat-treated further to achieve a final ceramic product.
With a modification and refinement of the gelcasting process it is possible to extend gelcasting technology to the manufacture of carbon and graphite products. Accordingly, the limitations related to current methods for fabricating graphite and carbon composites can be virtually eliminated.
It is an object of this invention to provide a method, and a product of that method, for manufacturing carbonaceous preforms using gelcasting technology.
It is another object of this invention to provide a method, and a product of that method, for manufacturing high density carbon and graphite composites using gelcast technology.
It is another object of this invention to provide gelcasting compositions, and methods of gelcasting, which allow the production of high density carbon and graphite composites.
It is another object of this invention to provide a method of manufacturing high density carbon and graphite composites having complex and intricate shapes.
It is still another object of this invention to provide a method of manufacturing high density carbon and graphite composites, which is less time consuming and results in an affordable product.
It is still yet another object of this invention to provide a method, and a product of that method, for manufacturing high density carbon and graphite composites using mesophase pitch powder or other polymer precursors without having an oxidative stabilization step.
These and other objects are accomplished by the present invention. The present invention is summarized in that it provides a novel method of manufacturing high density carbon and graphite composites utilizing a gelcasting method which allows the production of articles having complex and intricate shapes. Specifically, the present invention discloses a method for manufacturing high density carbon and graphite composites from a gelcast suspension comprising a polymeric carbon precursor, a solvent, a dispersant, an anti-foaming agent, a monomer system which is soluble in the solvent, and an initiator system. The suspension is formed by combining a volume of the polymeric carbon precursor with an appropriate volume of the solvent, the monomer system, the dispersant and the anti-foaming agent. The initiator solution is then mixed into the suspension and the suspension poured into a mold where it is heat polymerized to form a thermoplastic part. The thermoplastic part can then be further densified and heat-treated to produce a high density carbon or graphite composite.
In one embodiment of the present invention, the gelcast suspension is supplemented with additives to modify and enhance the properties of the composite material.
One advantage of the present invention is that it provides a method for manufacturing carbon or graphite composites which do not require the use of a hot press.
Another advantage of the present invention is that it provides a method for manufacturing carbon or graphite composites, or net shaped fiber (or particulate) reinforced polymeric parts, without the requirement of an oxidative stabilization step.
Yet another advantage of the invention is that it provides a method for manufacturing complex and intricately shaped thermoplastic parts which, when heat-treated, are further developed into near net-shaped monolithic carbon, carbon-carbon, or graphite parts, having superior physical characteristics.
Still yet another advantage is that the present invention provides a more affordable process for manufacturing carbon or graphite composites in a shorter period of time.
These and other objects and advantages of the invention are readily understood in view of the following figures and detailed description.